1. Field of the Invention
The invention relates to a technical field of low-voltage electrical apparatus, more particularly, relates to a current transformer used for supplying power to an electronic release.
2. The Related Art
In a power distribution system, a circuit breaker performs functions of connecting, breaking or carrying a rated operation current, the circuit breaker further performs a function of protecting fault currents such as a short circuit current or an overload current. When a short circuit occurs in a circuit, the circuit breaker can automatically cut off the circuit under the premise of not using an external power supply so that a reliable protection is achieved. A breaking device in the circuit breaker is used for realizing a breaking action. A current transformer supplies power to the breaking device. The power of the current transformer comes from a current flowing through a primary conductor of the circuit breaker, that is, a primary current.
FIG. 1 illustrates a structural diagram of a current transformer according to prior art. As shown in FIG. 1, the current transformer comprises a closed magnetic circuit 101. The closed magnetic circuit 101 includes laminated or wound soft magnetic metal sheets, riveting pieces 102 connect the soft magnetic metal sheets to form the closed magnetic circuit 101. The closed magnetic circuit 101 completely surrounds a primary conductor 107. For the purpose of match the shape of the primary conductor 107, a first part of the closed magnetic circuit 101 (the upper part shown in FIG. 1) is designed to have a corresponding shape. As shown in FIG. 1, the first part of the closed magnetic circuit 101 is arc-shaped to accommodate a circular primary conductor 107. A second part of the closed magnetic circuit 101 (the lower part shown in FIG. 1) serves as a magnetic core of a secondary winding 113. FIG. 2 illustrates a structural diagram of a secondary winding of a current transformer according to prior art. As shown in FIG. 2, a main structure of the secondary winding is an insulating framework 204. The insulating framework 204 is hollow to form a cavity 203. The second part of the closed magnetic circuit 101 passes through the cavity 203 (see FIG. 1). The insulating framework 204 is wound with a wire 205, and the wire 205 forms a coil. The number of turns of the coil may be set according to requirements. The wire 205 is covered by an insulating layer 201. The wire 205 leads two leads 206 extending out of the insulating layer 201. The leads 206 shown in FIG. 2 are the leads 115 on the secondary winding 113 shown in FIG. 1. Sheet-shaped structures 202 are formed on both ends of the insulating framework 204, and the sheet-shaped structure 202 isolates the magnetic circuit and the wire. As shown in the figure, the sheet-shaped structure 202 is extended outwardly from the insulating framework 204, the sheet-shaped structure 202 has a larger cross-sectional area than the insulating framework 204. A current transformer with such a structure has a good linear output characteristic when a primary current does not reach a large level of saturation of the magnetic material. When the primary current increases, a secondary current also increases in proportion so as to meet the requirements of power supply energy for the circuit breaker protection device.
Existing universal circuit breakers generally adopt a built-in structure, volume becomes a major factor that affects the performance of a current transformer. Due to the limitation of volume, the size of the current transformer cannot be increased infinitely. For small-shell circuit breakers, because of a small size of a small-shell circuit breaker, a shell of a current transformer therein is also small. Then a magnetic circuit volume of the current transformer and the number of turns of a coil on a secondary winding are limited. Under the condition that the number of turns of the coil is limited, the output energy of the secondary winding coil is small. The circuit breaker cannot achieve an automatic cut off of the circuit without the help of an external power supply under a condition when a short circuit transient current is small multiple of a minimum rated current of the circuit breaker (generally 2In˜3In). A tripping device is required to be driven by an energy outputted by the current transformer under a large multiple of the rated current. The application of the current transformer is thus limited.